SUMMARY

Theinterstellar medium
occupies the space among the stars. It is made up of cold (less than
100 K) gas, mostly atomic or molecular hydrogen and helium, and dust grains.
Interstellar dust is very effective at blocking our view of distant
stars, even though the density of the interstellar medium is very low.
The spatial distribution of interstellar matter is patchy. The general
diminution of starlight by dust is called extinction. In addition, the dust preferentially absorbs short-wavelength radiation, leading to a distinct reddening
of light passing interstellar clouds. Interstellar dust is thought to
be composed of silicates, graphite, iron, and “dirty ice.” Interstellar
dust particles are apparently elongated or rodlike. The polarization of starlight provides a means of studying them.

A nebula is a general term for any fuzzy bright or dark patch on the sky. Emission nebulae
are extended clouds of hot, glowing interstellar gas. Associated with
star formation, they result when hot O- and B-type stars heat and
ionize their surroundings. Studies of the emission lines produced by
excited nebular atoms allow astronomers to measure the nebula’s
properties. Some excited atomic states take so long to emit a photon
that the spectral lines associated with these transitions are never
seen in terrestrial laboratories, where collisions always knock the
atom into another energy state before it can emit any radiation. When
these lines are seen in nebular spectra, they are called forbidden
lines. Nebulae are often crossed by dark dust lanes, part of the larger
cloud from which they formed.

Dark dust clouds
are cold, irregularly shaped regions in the interstellar medium that
diminish or completely obscure the light from background stars.
Astronomers can learn about these clouds by studying the absorption
lines they produce in starlight that passes through them. Another way
to observe cold, dark regions of interstellar space is through 21-centimeterradiation.
Such radiation is produced whenever the electron in an atom of hydrogen
reverses its spin, changing its energy very slightly in the process.
This radio radiation is important because it is emitted by all cool
atomic hydrogen gas, even if the gas is undetectable by other means. In
addition, 21-cm radiation is not appreciably absorbed by the
interstellar medium, so radio astronomers making observations at this
wavelength can “see” to great distances.

The interstellar medium also contains many cold, darkmolecular clouds,
which are observed mainly through the radio radiation emitted by the
molecules they contain. Dust within these clouds probably both protects
the molecules and acts as a catalyst to help them form. As with other
interstellar clouds, hydrogen is by far the most common constituent,
but molecular hydrogen happens to be very hard to observe. Astronomers
usually study these clouds through observations of other “tracer”
molecules that are less common but much easier to detect. Often,
several molecular clouds are found close to one another, forming an
enormous molecular cloud complex millions of times more massive than the Sun.

SELF-TEST: TRUE OR FALSE?

1.Interstellar matter is quite evenly distributed throughout the Milky Way Galaxy. HINT

2.In
the vicinity of the Sun, the amount of mass in the form of interstellar
matter is comparable to the amount of mass in the form of stars. HINT

3.There is a lack of heavy elements in interstellar gas because they go into making interstellar dust. HINT

4.The
fact that starlight becomes polarized as it passes through the
interstellar medium tells us that interstellar dust particles are
spherical in shape. HINT

5.A typical region of dark interstellar space has a temperature of about 500 K. HINT

6.An emission nebula is a cloud of dust reflecting the light of a nearby group of stars. HINT

7.Emission nebulae display spectra almost identical to those of the stars embedded in them. HINT

8.Forbidden emission lines can occur in emission nebulae because the density of interstellar gas there is extremely low. HINT

9.Because
of the obscuration of visible light by interstellar dust, we can
observe stars only within a few thousand parsecs of Earth in any
direction. HINT

PROBLEMS

The number of squares preceding each problem indicates the approximate level of difficulty.

1.The
average density of interstellar gas within the Local Bubble is much
lower than the value mentioned in the text—in fact, it is roughly 103 hydrogen atoms/m3. Given that the mass of a hydrogen atom is 1.7 10-27 kg, calculate the total mass of interstellar matter contained within a Bubble volume equal in size to planet Earth. HINT

2.Assuming
the same average density as in the previous question, calculate the
total mass of interstellar hydrogen contained within a cylinder of
cross-sectional area 1 m2, extending from Earth to Alpha Centauri. HINT

3.Given
the average density of interstellar matter stated in Section 18.1,
calculate how large a volume of space would have to be compressed to
make a cubic meter of gas equal in density to air on Earth (1.2 kg/m3). HINT

4.Assuming a density of 3000 kg/m3, estimate the mass of the dust particle illustrated in Figure 18.3 (a). HINT

5.A
beam of light shining through a dense molecular cloud is diminished in
intensity by a factor of two for every 5 pc it travels. By how many
magnitudes is the light from a background star dimmed, if the total
thickness of the cloud is 60 pc? HINT

6.Interstellar
extinction is sometimes measured in magnitudes per kiloparsec (1 kpc =
1000 pc). Light from a star 1500 pc away is observed to be diminished
in intensity by a factor of 20 over and above the effect of the
inverse-square law. What is the average interstellar extinction along
the line of sight, in mag/kpc? HINT

7.Spectroscopic observations of a certain star reveal it to be a B2II giant, with absolute magnitude –6. (Secs. 17.4,17.7)
The star’s apparent magnitude is 14. Neglecting the effects of
interstellar extinction, calculate the distance to the star. If the
star’s distance is known (by other means) to be 5000 pc, calculate the
average extinction along the line of sight, in mag/kpc. (More Precisely 17-1) HINT

8.A star of apparent magnitude 10 lies 500 pc from Earth. If interstellar absorption results in an average extinction of
2 mag/kpc, calculate the star’s absolute magnitude and luminosity. HINT

9.A
star of known absolute magnitude 25 has apparent magnitude 10. If
interstellar absorption results in an average extinction of 2 mag/kpc,
calculate the star’s distance. (Note: This problem does not have an
algebraic solution. You wil have to solve it by numerical
means—essentially trial and error on a calculator.) HINT

10.To carry enough energy to ionize a hydrogen atom, a photon must have a wavelength of less than 9.12 10-8 m
(91.2 nm). Using Wien’s law, calculate the temperature a star must have
for the peak wavelength of its blackbody curve to equal this value. (Sec. 3.4) HINT

11.Estimate
the escape speeds near the edges of the four emission nebulae listed in
Table 18.1, and compare them with the average speeds of hydrogen nuclei
in those nebulae. (More Precisely 8-1) Do you think it is possible that the nebulae are held together by their own gravity? HINT

12.What would the mass of M8 have to be in order for its escape speed to equal its average molecular speed? HINT

13.If
a group of interstellar clouds along the line of sight have radial
velocities in the range 75 km/s (receding) to 50 km/s (approaching),
calculate the range of frequencies and wavelengths over which the
21.1-cm (1420 MHz) line of hydrogen will be observed. (Sec. 3.5) HINT

14.Calculate the radius of a spherical molecular cloud whose total mass equals the mass of the Sun. Assume a cloud density of 1012 hydrogen atoms per cubic meter. HINT

15.A cloud of atomic hydrogen has a radius of 1 pc and an average density 106
hydrogen atoms per cubic meter. Collisions between atoms ensure that,
at any instant, 3/4 of all atoms are in the upper (parallel spin)
state, as discussed in Section 18.4. The transition producing the 21-cm
line is very unlikely—the probability that any given atom in the upper
state will make the transition during any given second is about 3 10-15 (compare with 108
for the Ha transition). Use these figures, together with the Planck
formula for the energy of the photon emitted in the transition, to
estimate the total radio luminosity of the cloud. (Sec. 4.2) HINT

COLLABORATIVE EXERCISES

1.Exploring Density.
The interstellar medium has a very low density of about 1000 per cubic
kilometer. Estimate the population density of students in the tallest
dormitory on campus using units of students per cubic feet and compare
to the population density of the classroom. Explain your reasoning.

RESEARCHING ON THE WEB

To complete the following exercises, go to the online Destinations Module for Chapter 18 on the Companion Website for Astronomy Today 4/e.

1.Access
the "Messier Catalog" pages and determine which objects constitute the
majority of objects M1 through M10 and which objects for M90 through
M100.

2.Access the "Nebulae: Fuzzy Patches In Space" page and define the four types of nebulae.

PROJECTS

1.The
constellation Orion the Hunter is prominent in the evening sky of
winter. Its most noticeable feature is a short, straight row of three
medium-bright stars: the famous belt of Orion. A line of stars extends
from the eastmost star of the belt, toward the south. This line
represents Orion’s sword. Towards the bottom of the sword is the sky’s
most famous emission nebula, M42, the Orion Nebula. Observe the Orion
Nebula with your eye, with binoculars, and with a telescope. What is
its color? How can you account for this? With the telescope, try to
find the Trapezium, a grouping of four stars in the center of M42.
These are hot, young stars; their energy causes the Orion Nebula to
glow.

2.Observe
the Milky Way on a dark, very clear night. Is it a continuous band of
light across the sky or is it mottled? The parts of the Milky Way that
appear missing are actually dark dust clouds that are relatively near
the Sun. Identify the constellations in which you see these clouds.
Make a sketch and compare with a star atlas. Find other small clouds in
the atlas and try to find them with your eye or with binoculars.

SKYCHART III PROJECTS

The
SkyChart III Student Version planetarium program on which these
exercises are based is included as a separately executable program on
the CD in the back of this text.

1.One
of the most interesting nebula is the Great Orion Nebula, M42. Use
SkyChart III to locate Orion, and determine what time of the year it is
overhead at a convenient time for viewing. While the nebula is visible
to the unaided eye, it is much better with binoculars, and only gets
more interesting as you use larger and larger telescopes to observe it.

In addition to the Practice Problems and Destinations modules, the Companion Website at http://www.prenhall.com/chaisson
provides for each chapter an additional true-false, multiple choice,
and labeling quiz, as well as additional annotated images, animations,
and links to related Websites.